Fix PRINT-UNPRINTABLE-OBJECT

There is no Common Lisp operator named PRINT-UNPRINTABLE-OBJECT. Given the context, it is certain that PRINT-UNREADABLE-OBJECT (www.lispworks.com/documentation/lw51/CLHS/Body/m_pr_unr.htm) was meant instead.

lispguide.xml

@@ -2087,7 +2087,7 @@ Robert Brown
  </p>
  <CODE_SNIPPET>
  (defmethod print-object ((p person) stream)
- (print-unprintable-object (p stream :type t :identity t)
+ (print-unreadable-object (p stream :type t :identity t)
  (with-slots (first-name last-name) p
  (safe-format stream "~a ~a" first-name last-name))))
  </CODE_SNIPPET>